FIG. 11 is a conceptual diagram showing one example of an alternating current electric system. In FIG. 11, 10 is a power converter that can control alternating current power, like an inverter or converter, and 20 is an alternating current electric machine such as an alternating current motor or alternating current power supply.
The alternating current electric system is such that alternating current power is exchanged between the power converter 10 and alternating current electric machine 20 by a power conversion operation by the power converter 10 and a motor operation, generator operation, or the like, by the alternating current electric machine 20.
In the heretofore described configuration, a reactance component exists in the alternating current electric machine 20, and reactance also exists in a cable between the power converter 10 and alternating current electric machine 20. Also, reactance also exists in a reactor or filter acting as a part connected partway along the cable. In FIG. 11, 30 indicates the reactance component excluding the alternating current electric machine 20.
Next, FIG. 12 is a conceptual diagram showing a specific example of FIG. 11. In FIG. 12, 11 is a full-bridge type inverter acting as a 3-phase power converter, 111 to 116 are semiconductor switching elements that configure the inverter 11, 21 is an alternating current motor acting as an alternating current electric machine, 40 is a power supply (direct current power supply), and 50 is a capacitor of a direct current voltage unit (hereafter referred to as a main capacitor).
The alternating current electric system is such that, by three phases (U, V, and W phases) of alternating current power supplied to the alternating current motor 21 being controlled by the switching elements 111 to 116 of the inverter 11 being turned on or off, it is possible to regulate the generated torque and rotation speed of the alternating current motor 21.
As this kind of alternating current electric system is publicly known, a detailed description of the circuit configuration and operation will be omitted.
The heretofore described kind of alternating current electric system is such that a problem occurs when the drive of the power converter 10 is stopped. That is, there is a problem in that electromagnetic energy having the previously described reactance component flows into the power converter 10 when the power converter 10 is stopped.
To describe with the alternating current electric system of FIG. 12 as an example, when the drive of the inverter 11 is stopped, that is, when all the switching elements 111 to 116 of the inverter 11 are turned off, the following kind of problem occurs due to electromagnetic energy of the alternating current motor 21 flowing into the inverter 11.
Firstly, the voltage of the main capacitor 50 of the direct current voltage unit rises via a reflux diode in the inverter 11, and when the voltage exceeds the breakdown voltage of the main capacitor 50 or switching elements 111 to 116, those parts are destroyed. Countermeasures such as increasing the capacitance of the main capacitor 50 or raising the breakdown voltage of the main capacitor 50 and switching elements 111 to 116 are effective in avoiding the problem, but all involve an increase in cost, volume, and generated loss, and the like.
In response to this, there is a method whereby a series connection circuit of a semiconductor switching element and resistor (a dynamic braking circuit) is added in series with the main capacitor 50, and the dynamic braking circuit is caused to operate when the voltage applied to the main capacitor 50 becomes excessive, thus suppressing a rise in the voltage of the main capacitor 50. However, this method too is such that, owing to the addition of the dynamic braking circuit, an increase in cost and volume is unavoidable. In particular, it is often the case that the drive of the power converter is stopped when current is flowing only when there is an emergency, and it is markedly uneconomical to provide a dynamic braking circuit only for this purpose.
Next, when the alternating current motor 21 is, for example, a permanent magnet synchronous motor (PMSM, hereafter also referred to as a PM motor), it may happen that the no-load induced voltage (induced electromotive force) becomes higher than the voltage of the direct current voltage unit when the motor rotation speed is high, in which case current continues to flow into the direct current voltage unit even after the power converter 10 is stopped. In particular, when a battery is connected as a power supply to the direct current voltage unit, the battery may be destroyed when the current flowing into the direct current voltage unit becomes excessive.
In this case, in order to circumvent a state wherein the current flowing into the direct current voltage unit becomes excessive, it is conceivable that a direct current switch is provided between the battery and power converter, and the direct current switch is shut off. However, when current is flowing into an alternating current electric machine (the PM motor in the example), the electromagnetic energy of the reactance component still flows into the main capacitor 50, as a result of which an overvoltage is applied to the main capacitor 50.
Also, besides the inverter 11 shown in FIG. 12, a semiconductor switching element through which current flows in two directions and which can be shut off, like, for example, a matrix converter, may be connected to the alternating current electric machine 20 as the power converter 10 configuring the alternating current electric system. The matrix converter is such that, when put into a state wherein there is no possibility of current flowing in either direction by the switching element being turned off, the electromagnetic energy of the reactance component has nowhere to go, as a result of which an excessive voltage is momentarily applied to the switching element, and the switching element may be destroyed.
Herein, FIG. 13 is a waveform diagram showing the result of a simulation when the drive of the inverter 11 is stopped in the alternating current electric system of FIG. 12. With a PM motor being used as the alternating current motor 21, FIG. 13 shows the behavior of the voltage, current, and the like when the drive of the inverter 11 is stopped at a time t1 in a state in which the inverter 11 is carrying out a regeneration operation (a state in which power generated by the PM motor is being supplied to the inverter 11 side).
According to FIG. 13, an originally 400V voltage of the direct current voltage unit (an inverter direct current voltage in the drawing) rises to 800V from the time t1 onward. Because of this, when the breakdown voltage of the main capacitor 50 is lower than 800V, the main capacitor 50 is destroyed.
This kind of phenomenon occurs when current cannot be caused to flow from the inverter 11 side into the power supply 40 after the drive of the inverter 11 is stopped in the configuration of FIG. 12.
For example, when the power supply 40 is configured of a battery 41 and a relay connection point (hereafter referred to as a direct current switch) 42 of a direct current relay, as shown in FIG. 14, it is no longer possible for current caused by regenerative power from the PM motor to be caused to flow into the battery 41 via a reflux diode of the inverter 11, and an overvoltage is applied to the main capacitor 50.
FIG. 15 is a waveform diagram showing the result of a simulation when the drive of the inverter 11 is stopped in the alternating current electric system of FIG. 14, and is a case wherein a simulation of a flow of current into the power supply 40 is carried out.
With this simulation, there is a calculation of the behavior of the system when the PM motor is caused to rotate at high speed, the no-load induced voltage becomes higher than the voltage of the direct current voltage unit, the drive of the inverter 11 is stopped at the time t1 at which the inverter 11 is carrying out a regeneration operation, and the direct current switch 42 is shut off at a subsequent time t2.
According to FIG. 15, from the time t1 at which the drive of the inverter 11 is stopped onward, a current larger than that up to the time t1 flows into the battery 41, as shown by reference sign d, and when the current resistance of the battery 41 is lower than the value of this current, there is concern that the battery 41 will be destroyed.
Also, as the voltage of the direct current voltage unit rises considerably from the time t2 at which the direct current switch 42 is shut off onward, the breakdown voltage of the main capacitor 50 becomes a problem.
As one method of resolving the heretofore described problem, there is heretofore known technology disclosed in PTL 1.
The heretofore known technology of PTL 1 is such that when a main switch (corresponding to the direct current switch 42 in FIG. 14) of a circuit is opened for some reason, all the upper arm or lower arm switching elements of an inverter are put into an on-state, thus short-circuiting a motor stator coil, and avoiding a flow of electromagnetic energy from the motor into the inverter.
Because of this, it is possible to prevent a flow of an excessive current into the power supply side, or an overvoltage being applied to the main capacitor.